Isoolefin polymers are prepared in carbocationic polymerization processes. Of special importance is butyl rubber which is a copolymer of isobutylene with a small amount of isoprene. Butyl rubber is made by low temperature cationic polymerization that generally requires that the isobutylene have a purity of >99.5 wt % and the isoprene have a purity of >98.0 wt % to prepare high molecular weight butyl rubber.
The carbocationic polymerization of isobutylene and its copolymerization with comonomers like isoprene is mechanistically complex. See, e.g., Organic Chemistry, SIXTH EDITION, Morrison and Boyd, Prentice-Hall, 1084-1085, Englewood Cliffs, N.J. 1992, and K. Matyjaszewski, ed, Cationic Polymerizations, Marcel Dekker, Inc., New York, 1996. The catalyst system is typically composed of two components: an initiator and a Lewis acid. Examples of Lewis acids include AlCl3 and BF3. Examples of initiators include Brønsted acids such as HCl, RCOOH (wherein R is an alkyl group), and H2O. During the polymerization process, in what is generally referred to as the initiation step, isobutylene reacts with the Lewis acid/initiator pair to produce a carbenium ion. Following, additional monomer units add to the formed carbenium ion in what is generally called the propagation step. These steps typically take place in a diluent or solvent. Temperature, diluent polarity, and counterions affect the chemistry of propagation. Of these, the diluent is typically considered important.
Industry has generally accepted widespread use of a slurry polymerization process (to produce butyl rubber, polyisobutylene, etc.) in the diluent methyl chloride. Typically, the polymerization process extensively uses methyl chloride at low temperatures, generally lower than −90° C., as the diluent for the reaction mixture. Methyl chloride is employed for a variety of reasons, including that it dissolves the monomers and aluminum chloride catalyst but not the polymer product. Methyl chloride also has suitable freezing and boiling points to permit, respectively, low temperature polymerization and effective separation from the polymer and unreacted monomers. The slurry polymerization process in methyl chloride offers a number of additional advantages in that a polymer concentration of approximately 26% to 37% by volume in the reaction mixture can be achieved, as opposed to the concentration of only about 8% to 12% in solution polymerization. An acceptable relatively low viscosity of the polymerization mass is obtained enabling the heat of polymerization to be removed more effectively by surface heat exchange. Slurry polymerization processes in methyl chloride are used in the production of high molecular weight polyisobutylene and isobutylene-isoprene butyl rubber polymers. Likewise polymerizations of isobutylene and para-methylstyrene are also conducted using methyl chloride. Similarly, star-branched butyl rubber is also produced using methyl chloride.
However, there are a number of problems associated with the polymerization in methyl chloride, for example, the tendency of the polymer particles in the reactor to agglomerate with each other and to collect on the reactor wall, heat transfer surfaces, impeller(s), and the agitator(s)/pump(s). The rate of agglomeration increases rapidly as reaction temperature rises. Agglomerated particles tend to adhere to and grow and plate-out on all surfaces they contact, such as reactor discharge lines, as well as any heat transfer equipment being used to remove the exothermic heat of polymerization, which is critical since low temperature reaction conditions must be maintained.
The commercial reactors typically used to make these rubbers are well mixed vessels of greater than 10 to 30 liters in volume with a high circulation rate provided by a pump impeller. The polymerization and the pump both generate heat and, in order to keep the slurry cold, the reaction system needs to have the ability to remove the heat. An example of such a continuous flow stirred tank reactor (“CFSTR”) is found in U.S. Pat. No. 5,417,930, incorporated by reference, hereinafter referred to in general as a “reactor” or “butyl reactor”. In these reactors, slurry is circulated through tubes of a heat exchanger by a pump, while boiling ethylene on the shell side provides cooling, the slurry temperature being determined by the boiling ethylene temperature, the required heat flux and the overall resistance to heat transfer. On the slurry side, the heat exchanger surfaces progressively accumulate polymer, inhibiting heat transfer, which would tend to cause the slurry temperature to rise. This often limits the practical slurry concentration that can be used in most reactors from 26 to 37 volume % relative to the total volume of the slurry, diluent, and unreacted monomers. The subject of polymer accumulation has been addressed in several patents (such as U.S. Pat. Nos. 2,534,698, 2,548,415, 2,644,809). However, these patents have unsatisfactorily addressed the myriad of problems associated with polymer particle agglomeration for implementing a desired commercial process.
U.S. Pat. No. 2,534,698 discloses, inter alia, a polymerization process comprising the steps in combination of dispersing a mixture of isobutylene and a polyolefin having 4 to 14 carbon atoms per molecule, into a body of a fluorine substituted aliphatic hydrocarbon containing material without substantial solution therein, in the proportion of from one-half part to 10 parts of fluorine substituted aliphatic hydrocarbon having from one to five carbon atoms per molecule which is liquid at the polymerization temperature and polymerizing the dispersed mixture of isobutylene and polyolefin having four to fourteen carbon atoms per molecule at temperatures between −20° C. and −164° C. by the application thereto a Friedel-Crafts catalyst. However, '698 teaches that the suitable fluorocarbons would result in a biphasic system with the monomer, comonomer and catalyst being substantially insoluble in the fluorocarbon making their use difficult and unsatisfactory.
U.S. Pat. No. 2,548,415 discloses, inter alia, a continuous polymerization process for the preparation of a copolymer, the steps comprising continuously delivering to a polymerization reactors a stream consisting of a major proportion of isobutylene and a minor proportion isoprene; diluting the mixture with from ½ volume to 10 volumes of ethylidene difluoride; copolymerizing the mixture of isobutylene isoprene by the continuous addition to the reaction mixture of a liquid stream of previously prepared polymerization catalyst consisting of boron trifluoride in solution in ethylidene difluoride, maintaining the temperature between −40° C. and −103° C. throughout the entire copolymerization reaction . . . . '415 teaches the use of boron trifluoride and its complexes as the Lewis acid catalyst and 1,1-difluoroethane as a preferred combination. This combination provides a system in which the catalyst, monomer and comonomer are all soluble and yet still affords a high degree of polymer insolubility to capture the benefits of reduced reactor fouling. However, boron trifluoride is not a preferred commercial catalyst for butyl polymers for a variety of reasons.
U.S. Pat. No. 2,644,809 teaches, inter alia, a polymerization process comprising the steps in combination of mixing together a major proportion of a monoolefin having 4 to 8, inclusive, carbon atoms per molecule, with a minor proportion of a multiolefin having from 4 to 14, inclusive, carbon atoms per molecule, and polymerizing the resulting mixture with a dissolved Friedel-Crafts catalyst, in the presence of from 1 to 10 volumes (computed upon the mixed olefins) of a liquid selected from the group consisting of dichlorodifluoromethane, dichloromethane, trichloromonofluormethane, dichloromonofluormethane, dichlorotetrafluorethane, and mixtures thereof, the monoolefin and multiolefin being dissolved in said liquid, and carrying out the polymerization at a temperature between −20° C. and the freezing point of the liquid. '809 discloses the utility of chlorofluorocarbons at maintaining ideal slurry characteristics and minimizing reactor fouling, but teaches the incorporation of diolefin (i.e. isoprene) by the addition of chlorofluorocarbons (CFC). CFC's are known to be ozone-depleting chemicals. Governmental regulations, however, tightly controls the manufacture and distribution of CFC's making these materials unattractive for commercial operation.
Additionally, Thaler, W. A., Buckley, Sr., D. J., High Molecular-Weight, High Unsaturation Copolymers of Isobutylene and Conjugated Dienes, 49(4) Rubber Chemical Technology, 960 (1976), discloses, inter alia, the cationic slurry polymerization of copolymers of isobutylene with isoprene (butyl rubber) and with cyclopentadiene in heptane.
Therefore, finding alternative diluents or blends of diluents to create new polymerization systems that would reduce particle agglomeration and/or reduce the amount of chlorinated hydrocarbons such as methyl chloride is desirable. Such new polymerization systems would reduce particle agglomeration and reactor fouling without having to compromise process parameters, conditions, or components and/or without sacrificing productivity/throughput and/or the ability to produce high molecular weight polymers. Additionally, finding new polymers associated with the aforementioned processes would help supply the world's increasing demand for elastomers and/or also provide for new end use applications.
Hydrofluorocarbons (HFC's) are chemicals that are currently used as environmentally friendly refrigerants because they have a very low (even zero) ozone depletion potential. Their low ozone depletion potential is thought to be related to the lack of chlorine. The HFC's also typically have low flammability particularly as compared to hydrocarbons and chlorinated hydrocarbons.
Isobutylene/isoprene copolymerizations have been conducted under a variety of polymerizations conditions including different monomer feed ratios, temperatures, catalysts, and solvents or diluents. A number of these systems have been described. See e.g. Cationic Polymerizations of Olefins: A Critical Inventory, J. P. Kennedy, (10-12 and 86-137) Wiley-Interscience, New York, 1972, and Carbocationic Polymerization, J. P. Kennedy, E. Marechal, Wiley-Interscience, New York, 1982.
The copolymerization of isobutylene and isoprene has been conducted in solution and in slurry. Solution polymerizations often employ chlorinated hydrocarbons or mixtures of hydrocarbons and chlorinated hydrocarbons to serve as a solvent for the monomers, catalyst and the prepared copolymers. A typical example is ethyl chloride/hexane mixtures. In slurry copolymerization, a chlorinated hydrocarbon, usually methyl chloride, is used as a solvent for the monomers and catalyst, but the copolymer produced is insoluble in the diluent.
The sequence distribution, characterization of the arrangement of the monomer units along polymer chain, of the prepared copolymers is influenced by the polymerization conditions as well as the intrinsic reactivity of the comonomers employed. The sequence distribution of a copolymer may be expressed in terms of combinations of adjacent structural units. For example, characterizable sequences of two monomer units are called diads. Three monomer unit sequences are called triads. Four monomer unit sequences are called tetrads and so forth. Copolymers prepared under different conditions with the same comonomer incorporation may exhibit differences in their sequence distributions as expressed by the diad (or triad, etc.) fractions in the copolymer chain. Sequence distributions and comonomer incorporation are mathematically linked by probability statistics because of the competitive nature of the chemical events involved in copolymerization. A parameter that aids in the characterization of this relationship is the reactivity ratio, a ratio of the rate constants of homopropagation (adding a like monomer) to cross propagation (adding an unlike monomer). Copolymers with the same comonomer incorporation, but with different sequence distributions often exhibit different physical properties. See e.g. Chemical Microstructure of Polymer Chains, J. L. Koenig, Wiley-Interscience, New York, 1980, and Polymer Sequence Determination: Carbon-13 NMR Method, J. C. Randall, Academic Press, 1977. An extreme, but clarifying example is the comparison of the physical attributes of random and block copolymers.
It is generally known that conjugated dienes are less reactive than isobutylene in carbocationic copolymerization systems. Of the known linear conjugated dienes, isoprene is one of the more reactive dienes in copolymerization with isobutylene. This tendency towards lower reactivity of the conjugated diene is expressed in the sequence distribution of the prepared copolymers. At a given copolymer composition, isoprene units do not exhibit a tendency to follow other isoprene units in the copolymer chain. Consequently, BII (B=isobutylene, I=isoprene), IIB and III triad fractions are relatively low than compared to systems with more reactive comonomers.
Because isobutylene/isoprene copolymerations are often conducted in chlorinated hydrocarbons or mixtures of hydrocarbons and chlorinated hydrocarbons, the degree to which the sequence distribution can be varied is quite limited. Expression of this limitation is found by examination of the known reactivity ratios of isoprene for isobutylene/isoprene copolymerizations See e.g., J. E. Puskas, “Carbocationic Polymerizations” in Encyclopedia of Polymer Science and Technology, (DOI: 10.1002/0471440264.pst040) John Wiley & Sons, New York, 2003. Values for isoprene reactivity ratios, rIP, under a variety of polymerization conditions fall below 1.4 indicating a narrow range of available isoprene centered triad fractions (BII, IIB and III) in the prepared copolymers. Finding a polymerization system in which different concentrations of isoprene centered triad fraction can be prepared at a given comonomer incorporation is desirable for preparing copolymers suitable for the simultaneous introduction of crosslinking sites and functional groups.
Other background references include WO 02/34794 that discloses a free radical polymerization process using hydrofluorocarbons. Other background references include DE 100 61 727 A, WO 02/096964, WO 00/04061, U.S. Pat. Nos. 5,624,878, 5,527,870, and 3,470,143.